Continental breakup involves a transition from rapid, fault-controlled syn-rift subsidence to relatively slow, post-breakup subsidence induced lithospheric cooling. Yet the stratigraphic record of many rifted margins contain syn-breakup unconformities, indicating episodes of uplift and erosion interrupt this transition. This uplift has been linked to mantle upwelling, depth-dependent extension, and/or isostatic rebound. Deciphering the breakup processes recorded by these unconformities and their related rock record is difficult because associated erosion commonly removes the strata that help constrain the onset and duration of uplift. We examine three major breakup-related unconformities and intervening rock record in the Lower Cretaceous succession of the Gascoyne and Cuvier margins, offshore NW Australia, using seismic reflection and borehole data. These data show the breakup unconformities are disconformable (non-erosive) in places and angular (erosive) in others. Our recalibration of palynomorph ages from rocks underlying and overlying the unconformities shows: (i) the lowermost unconformity developed between 134.98–133.74 Ma (Intra-Valanginian), probably during the localisation of magma intrusion within continental crust and consequent formation of continent-ocean transition zones (COTZ); (2) the middle unconformity formed between ~134–133 Ma (Top Valanginian), possibly coincident with breakup of continental crust and generation of new magmatic (but not oceanic) crust within the COTZs; and (iii) the uppermost unconformity likely developed between ~132.5–131 Ma (i.e. Intra-Hauterivian), coincident with full breakup of continental lithosphere and the onset of seafloor spreading. During unconformity formation, uplift was focused along the continental rift flanks, likely reflecting landward flow of lower crustal and/or lithospheric mantle from beneath areas of localised extension towards the continent (i.e. depth-dependent extension). Our work supports the growing consensus that the ‘breakup unconformity’ is not always a single stratigraphic surface marking the onset of seafloor spreading; multiple unconformities may form and reflect a complex history of uplift and subsidence during the development of continent-ocean transition.
The stratigraphic record of continental breakup, offshore NW Australia